There is a growing need for low cost, light-weight, low profile, readily mass-producible, high aperture-efficiency antennas of useful bandwidth in a variety of mass market applications.
The desirable characteristics of low cost, light-weight, low profile and mass producibility are provided in general by printed circuit antennas. The simplest forms of printed circuit antennas are "microstrip" antennas wherein flat conductive elements are spaced from a single essentially continuous ground element by a single dielectric sheet of uniform thickness. Such antennas are easily constructed from one layer of double clad circuit board material. Microstrip antennas with increased aperture efficiency and increased bandwidth would be very desirable.
One type of microstrip antenna utilizes radiating monopoles, each of which produce an omnidirectional radiation pattern in the plane of the antenna surface. Such an antenna is disclosed in U.S. Pat. No. 3,377,592 wherein short sections of otherwise uniform microstrip transmission lines are displaced in one direction from the centerline of the transmission line at intervals of one wavelength. All the outside corners of any one transmission line acquire the same charge simultaneously to produce monopoles and a radiation pattern that has a principal lobe that is tangential to the surface of the antenna.
A second type of microstrip antenna utilizes thin conductive resonant dipoles radiator elements, each of which produces a radiation pattern having a principal lobe broadside (perpendicular) to the antenna surface. Each of such dipole radiator elements has two orthogonal coordinates that respectively define E and H planes of electromagnetic radiation for that radiator element. The E coordinate dimension of each radiator element is approximately one-half the dielectric wavelength ##EQU2## where .lambda..omicron. is the free space wavelength, .epsilon..sub.r is the relative dielectric constant and .mu..sub.r is the relative permeability of the dielectric sheet. The dielectric sheet is generally ##EQU3## thick with the preferred range being ##EQU4## In an antenna it is desirable that such radiator elements radiate in a predetermined amplitude and phase relationship with respect to each other. The amplitude relationship may be a uniform illumination wherein all radiator elements contribute equally to a radiation pattern. Alternatively, the amplitude relationship may be a tapered distribution. The radiator elements should radiate in phase with respect to each other to create a broadside beam. An off-broadside beam may be created by having a progressive phase shift along rows or columns of radiator elements.
One class of microstrip antennas utilizing resonant dipole radiator elements employs capacitative coupling of energy to radiator elements. Such an antenna is disclosed in U.S. Pat. No. 3,016,536 wherein rectangular resonant dipole radiator elements are distributed on a broad surface. The E coordinate dimension of each radiator element is approximately ##EQU5## The H coordinate dimension of each radiator element is considerably less than the E coordinate dimension. Such radiator elements form collinear arrays in the E coordinate direction with capacitative coupling between radiator elements for energy transfer. The center dipole of each collinear array consists of a pair of quarter wavelength radiator elements that form a balanced center-fed dipole. Several center-fed dipoles and their respective collinear arrays are driven from a balanced line to provide a two dimensional planar array. Such an antenna requires a balanced drive, has a poor aperture efficiency and a narrow bandwidth. The antenna has a rather large thickness because it is designed to use the ground plane as a reflector.
Another example of resonant dipole microstrip antennas utilizing capacitative coupling is contained in EMI-Varian Limited Bulletin PA2 11/73, entitled "Printed Antennae 2-36 GHz". In such an antenna the radiator elements are capacitatively coupled at various spacings to one or more feed lines running parallel to their E coordinate. The disclosed antenna has demonstrated low aperture efficiencies and poor side lobe control.
A second class of microstrip antennas utilizing resonant dipole radiator elements employs conductive coupling of energy to radiator elements. Antennas of this class are disclosed in U.S. Pat. No. 3,803,623 (Charlot) and U.S. Pat. No. 3,811,128 (Munson) and by Munson (I.E.E.E. Transactions on Antennas and Propagation, January, 1974, pp. 74-78). The E coordinate dimension of the radiator elements is approximately ##EQU6## The H coordinate dimension may be greater or lesser than the E coordinate dimension. The individual input impedance of such radiator elements at frequencies around resonance is typically in the convenient range of 50 to 150 ohms depending on element dimensions and dielectric substrate characteristics.
A corporate feed network distributes energy between the transmission line and a plurality of microstrip radiator elements. A corporate feed network in microstrip comprises an interconnected pattern of thin conductive strips which connect the radiator elements into arrays. A terminal on the corporate feed network of an array serves for connection to a transmission line. Such a terminal may be connected directly to the transmission line or connected indirectly to the transmission line through additional corporate feed network strips.
A corporate feed network may be provided by a sequence of power dividers and tapered feed line sections or other impedance transformers which serve to distribute the desired amount of energy directly from (to) the transmission line to (from) each radiator element. The lengths of the feed line sections determine the phase relationship between the transmission line and each radiator element and thus control the phase relationship between radiator elements. Two dimensional arrays of up to four or possibly eight radiator elements interconnected by a corporate feed network can be designed to produce a good aperture efficiency in the range of 90 percent based on ground element area. For arrays of greater numbers of radiator elements a decreased aperture efficiency is observed with conventional corporate feed because the corporate feed network becomes increasingly more extensive. The more extensive feed network necessitates increasing the spacing between the radiator elements, with such increased radiator element spacing in turn significantly reducing the aperture efficiency. such proliferating feed lines also become lengthy which increases feed line losses. The proliferating feed lines often have lengths of various multiples of dielectric wavelengths such that slight changes in frequency produce undesirable phase shifts between radiator elements.